Measurement sensor for determining an oxygen concentration in a gas mixture

ABSTRACT

A probe is described for determining an oxygen concentration in a gas mixture, in particular in the exhaust gas of internal combustion engines, having a Nernst measuring cell, which has a first electrode (Nernst electrode) which is exposed to the gas mixture to be measured via a diffusion barrier, a second electrode (reference electrode) which is exposed to a reference gas, and a solid electrolyte body arranged between the first and the second electrode, and having a pump cell, which has a first electrode (inner pump electrode) which is exposed to the gas mixture via the diffusion barrier, a second electrode (outer pump electrode) which is exposed to the gas mixture, and a solid electrolyte body arranged between the first and the second electrode. The Nernst electrode and the inner pump electrode are connected at least in some sections via a joint supply conductor to a circuit arrangement for controlling and evaluating the probe. A joint supply conductor resistor of the Nernst electrode and of the inner pump electrode is formed by a loaded voltage divider whose individual resistors are arranged so that the negative feedback of a Nernst voltage circuit and of a pump voltage circuit is optimized, in particular maximized.

FIELD OF THE INVENTION

The present invention relates to a probe for determining an oxygenconcentration in a gas mixture, in particular in the exhaust gas ofinternal combustion engines having the features set forth in thepreamble of claim 1.

BACKGROUND INFORMATION

Previously proposed probes determine the oxygen concentration in theexhaust gas of internal combustion engines and are used to influence thesetting of the fuel air mixture during operation of the engine. Thefuel/air mixture may be in the rich range, i.e., there is an excess offuel in stoichiometric terms, so that only a small quantity of oxygenrelative to other partly unburned components is present in the exhaustgas. In the lean range, in which there is a greater quantity of oxygenrelative to the air in the fuel/air mixture, the oxygen concentration inthe exhaust gas is correspondingly high. If the fuel/air mixture is ofstoichiometric composition, both the amount of fuel and the amount ofoxygen in the exhaust gas are reduced.

Lambda sensors which detect a lambda value>1 in the lean range, a lambdavalue<1 in the rich range, and a lambda value=1 in the stoichiometricrange and which are used to determine the oxygen concentration inexhaust gas are known. In this case, the lambda sensor supplies adetection voltage in a known manner, which is conveyed to a circuitarrangement. In known probes, with the help of the circuit arrangementthe detection voltage is converted into a pump voltage for a pump cell,which is also a component of the probe and is exposed to the exhaustgas. The pump cell, in which oxygen ions are pumped from an inner pumpelectrode to an outer pump electrode or vice versa based on the oxygenconcentration present. Depending on whether the lambda sensor detects arich range, i.e., a lambda value<1, or a lean range, i.e., a lambdavalue>1, the circuit arrangement determines whether the outer pumpelectrode, which is connected to an active input of the circuitarrangement, is connected as a cathode or as an anode. The inner pumpelectrode of the pump cell is connected to ground, so that at the pumpcell an anodic limit current flows in the case of rich measured gas or acathodic limit current flows in the case of a lean measured gas. In thecase of stoichiometric operation, i.e., if the lambda value=1, the pumpvoltage is close to 0, so that no limit current flows.

The detection voltage of the probe is determined via a Nernst measuringcell, which determines the difference between the oxygen concentrationat a Nernst electrode and that at a reference electrode. The referenceelectrode is connected to a constant current source, while the Nernstelectrode is connected to ground. As a result, the detection voltage isbased correspondingly on the difference between the respective oxygenconcentrations.

Because the Nernst electrode and the inner pump electrode of the probeare connected to ground, it is known that they can be connected to thecircuit arrangement via a joint supply conductor. In this case, theelectrodes are initially contacted inside the probe to separate printedconductors, which then come together inside the probe at a contact pointto form the joint supply conductor.

By detecting the pump current of the pump cell required to maintain λ=1in a measuring space (hollow space) of the probe, it is possible todetermine whether the fuel/air mixture used to operate the internalcombustion engine is a rich or a lean mixture. If there is a change-overfrom a rich range to a lean range or vice versa, the pump current dropsor increases, respectively. If the engine is being operated in thestoichiometric range, i.e., with a lambda value=1, the pump current hasa jump point that marks the transition from the lean range to the richrange and vice versa, respectively.

Referring to FIG. 4, there is seen a conventional connectivity between agas probe and an operational amplifier. In known probes, it isdisadvantageous that because the supply conductor of the Nernstelectrode and the inner pump electrode is shared, at least in somesections, their joint supply conductor resistor, which is not only partof the Nernst voltage circuit of the Nernst measuring cell but also partof the pump voltage circuit of the pump cell, causes coupling, which hasan impact on lambda=1 ripple. This minimizes the counterswings andoverswings in voltage that may occur in the event of a jump response inresponse to a transition from the rich range to the lean range.

SUMMARY OF THE INVENTION

By contrast, the probe according to the present invention has theadvantage that negative feedback of the pump voltage circuit and theNernst voltage circuit is optimized. Because a joint supply conductorresistor of the Nernst electrode and of the inner pump electrode isformed by a loaded voltage divider whose individual resistors arearranged so that negative feedback of a Nernst voltage circuit and of apump voltage circuit is increased, the lambda=1 ripple can be reduced.The individual resistors are arranged so that when the detection voltageof the Nernst measuring cell transitions from the lean range to the richrange or vice versa, this produces a result via the jump point thattriggers an anodic or cathodic limit current, respectively, via the pumpcell, so that negative feedback via the joint supply conductor sectionof the Nernst measuring cell and the pump cell can be achieved.

According to a preferred embodiment of the present invention, anadditional external resistor is connected in series to the joint supplyconductor section of the Nernst measuring cell and the pump cell. Thanksto this additional external resistor, the total resistance of the jointsupply conductor section is increased, so that at the constant currentat which the Nernst measuring cell is operated the detection voltage isgreater, so that the influence of negative feedback is increased by thecathodic or alternatively anodic limit current, which also flows throughthe additional resistor.

According to a further preferred embodiment of the present invention, across section of the joint supply conductor section is reduced. Reducingthe cross section is another way to increase the resistance value of thejoint supply conductor section, so that this is also a straightforwardway of increasing negative feedback between the Nernst voltage circuitand the pump voltage circuit.

According to a further preferred embodiment of the present invention,the contact point where the printed conductor of the inner pumpelectrode meets the printed conductor of the Nernst electrode is movedspatially as close as possible to the electrodes, so that the length ofthe joint supply conductor section increases, so that the resistance ofthis joint supply conductor section is also increased by a definedamount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through the head of a probe.

FIG. 2 shows an equivalent circuit diagram of a joint supply conductorof a Nernst electrode and an inner pump electrode of the probe.

FIG. 3a shows one embodiment for influencing the resistances of thejoint supply conductor according to FIG. 2.

FIG. 3b shows a second embodiment for influencing the resistances of thejoint supply conductor according to FIG. 2.

FIG. 4 shows connectivity between a gas probe and an operationalamplifier.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows a section through a measuring head of a probe 10. Probe 10is designed as a planar broadband probe and includes a plurality ofindividual layers which are arranged one above the other and may, forexample, be structured via film casting, punching, screen printing,lamination, cutting, vitrification or other processes. The processesused to achieve layer structure will not be discussed in greater detailin the context of the present description, as this is known.

Probe 10 is used to determine an oxygen concentration in the exhaust gasof internal combustion engines, so as to generate a control signal forsetting a fuel/air mixture used to operate the internal combustionengine. Probe 10 has a Nernst measuring cell 12 and a pump cell 14.Nernst measuring cell 12 has a first electrode 16 (Nernst electrode) anda second electrode 18 (reference electrode), between which a solidelectrolyte 20 is arranged. Electrode 16 is exposed to exhaust gas 24 tobe measured via a diffusion barrier 22. Probe 10 has a measuring opening26 to which exhaust gas 24 can be supplied. Diffusion barrier 22 extendsat the base of measuring opening 26, a hollow space 28 being formedwithin which electrode 16 is arranged. Electrode 18 of Nernst measuringcell 12 is arranged in a reference air channel 30 and exposed to areference gas, e.g., air, present in reference air channel 30. Solidelectrolyte 20 is made of, for example, yttrium-oxide-stabilizedzirconium oxide, while electrodes 16 and 18 are made of, for example,platinum.

Probe 10 is connected to a circuit arrangement 32 (only indicated here)which evaluates the signals of probe 10 and controls the probe.Electrodes 16 and 18 are connected to inputs 34 and 36, respectively, ofcircuit arrangement 32, to which detection voltage U_(D) of Nernstmeasuring cell 12 is applied.

Pump cell 14 includes a first electrode 38 (inner pump electrode) and asecond electrode 40 (outer pump electrode) between which a solidelectrolyte 42 is arranged. Solid electrolyte 42 is in turn made of, forexample, a yttrium-oxide-stabilized zirconium oxide, while electrodes 38and 40 may in turn be made of platinum. Electrode 38 is also arranged inhollow space 28 and is thus also exposed to exhaust gas 24 via diffusionbarrier 22. Electrode 40 is covered by a protective layer 44, which isporous, so that electrode 40 is directly exposed to exhaust gas 24.Electrode 40 is connected to an input 46 of circuit arrangement 32,while electrode 38 is connected to electrode 16 and, along with it, isconnected jointly to input 34 of circuit arrangement 32. This jointsupply conductor of electrodes 16 and 38 connected to circuitarrangement 32 will be discussed in greater detail below with referenceto FIGS. 2 and 3.

Probe 10 also includes a heating device 49 which is formed by ameandering heating element and to which a heating voltage U_(H) can beapplied.

Probe 10 functions as follows:

Exhaust gas 24 enters hollow space 28 via measuring opening 26 anddiffusion barrier 22 and is thus present at electrode 16 of Nernstmeasuring cell 12 and electrode 38 of pump cell 14. A difference in theoxygen concentration at electrode 16 and that at electrode 18, which isexposed to the reference gas, arises based on the oxygen concentrationin the exhaust gas to be measured. Electrode 16 is connected to acurrent source of circuit arrangement 32, which supplies a constantcurrent, via terminal 34. A specific detection voltage U_(D) (Nernstvoltage) arises based on a difference between the oxygen concentrationpresent at electrode 16 and that at electrode 18. Here, Nernst measuringcell 12 functions as a lambda sensor that detects whether a high oxygenconcentration or a low oxygen concentration is present in exhaust gas24. It is clear from the oxygen concentration whether the fuel/airmixture used to operate the internal combustion engine is a rich or alean mixture. If there is a change-over from the rich range to the leanrange or vice versa, detection voltage U_(D) drops or increases,respectively. With stoichiometric operation, i.e., with a lambdavalue=1, detection voltage U_(D) has a jump point that marks thetransition from a lean range to a rich range or vice versa,respectively.

With the help of circuit arrangement 32, detection voltage U_(D) is usedto determine pump voltage U_(P), which is applied to pump cell 14between its electrodes 38 and 40, respectively. Pump voltage U_(P) isnegative or positive based on whether detection voltage U_(D) signalsthat the fuel/air mixture is in the rich or lean range, so thatelectrode 40 is connected either as a cathode or as an anode.Accordingly, a pump current I_(P) is established and can be measured viaa measuring device of circuit arrangement 32. With the help of pumpcurrent I_(P), oxygen ions are pumped from electrode 40 to electrode 38or vice versa. Measured pump current I_(P) is used to control a devicefor setting the fuel/air mixture used to operate the internal combustionengine.

The detection voltage circuit (Nernst voltage circuit) and the pumpvoltage circuit are coupled to circuit arrangement 32 via the jointsupply conductor of electrodes 16 and 38, respectively. In FIG. 2, anequivalent circuit diagram illustrating how electrodes 16 and 38 areconnected to circuit arrangement 32 is shown. It is clear from theequivalent circuit diagram that electrode 38 is initially connected to acontact point 52 via a printed conductor section 50. Electrode 16 isalso connected to contact point 52 via a printed conductor section 54. Aprinted conductor section 56 connects contact point 52 to input 34 ofcircuit arrangement 32. Contact point 52 is arranged inside probe 10 andis located at a geometric distance a from electrodes 16 and 38,respectively, indicated here. A geometric distance b for joint supplyconductor section 56 of electrodes 16 and 38 results, corresponding tosection a.

Conductor section 50 has an internal resistor R1, conductor section 54has an internal resistor R2, and conductor section 54 has an internalresistor R3. Resistors R1, R1, and R3 form a loaded voltage divider, theconstant current applied to Nernst measuring cell 12 flowing viaconductor sections 54 and 56, while pump current I_(P) flows viaconductor sections 50 and 56.

FIG. 3a shows a first embodiment variant for arranging the loadedvoltage divider formed by resistors R1, R2, and R3. An additionalresistor R4 is connected between terminal 34 and circuit arrangement 32(FIG. 1). This effectively increases the resistance value of jointsupply conductor section 56 of electrodes 16 and 38, the resistancebeing the sum of resistances R3 and R4. Thanks to this greaterresistance R3+R4, the Nernst voltage increases given the constantcurrent applied to Nernst measuring cell 12 via circuit arrangement 32.

According to the embodiment variant shown in FIG. 3b, contact point 52is moved geometrically closer to electrodes 16 and 38, so that thelength of joint supply conductor section 56, i.e., distance b′ betweencontact point 52 and terminal 34, is increased. As a result, theresistance value of resistor R3 is increased relative to the initialembodiment shown in FIG. 2. In particular, this causes supply conductorresistor R3 to have a positive temperature coefficient.

According to a further embodiment variant (not shown), joint supplyconductor section 56 between contact point 52 and terminal 34 may have asmaller cross section than that of sections 50 and 54, respectively, sothat as a result the resistance value of resistor R3 increases.

What is claimed is:
 1. A probe for determining an oxygen concentrationin a gas mixture, comprising: a Nernst measuring cell including: aNernst electrode exposed to the gas mixture to be measured via adiffusion barrier, a reference electrode exposed to a reference gas, anda solid electrolyte body arranged between the Nernst electrode and thereference electrode; a pump cell including: an inner pump electrodeexposed to the gas mixture via the diffusion barrier, an outer pumpelectrode exposed to the gas mixture, and a solid electrolyte bodyarranged between the inner pump electrode and the outer pump electrode;a joint supply conductor section through which the Nernst electrode andthe inner pump electrode are connected to a circuit arrangement forcontrolling and evaluating the probe; and a loaded voltage dividerincluding a plurality of resistors that are arranged such that anegative feedback of a Nernst voltage circuit and of a pump voltagecircuit is optimized, the plurality of resistors including a jointsupply conductor resistor associated with the Nernst electrode and theinner pump electrode; wherein magnitudes of the plurality of resistorsare chosen so as to reduce a rippling effect at a stoichiometric point.2. The probe according to claim 1, wherein: the gas mixture correspondsto an exhaust gas of an internal combustion engine.
 3. The probeaccording to claim 1, further comprising: an additional externalresistor connected in series to the joint supply conductor section. 4.The probe according to claim 1, wherein: a cross section of the jointsupply conductor section is minimized.
 5. The probe according to claim1, further comprising: printed conductor sections via which the Nernstelectrode and the inner pump electrode are connected to a contact point,wherein: the cross section of the joint supply conductor section issmaller than a cross section of the printed conductor sections.
 6. Theprobe according to claim 1, wherein: the Nernst electrode and the innerpump electrode are connected to the circuit arrangement via the jointsupply conductor section by a contact point, and the contact point islocated directly downstream of the Nernst electrode and the inner pumpelectrode at a first distance such that a second distance of the jointsupply conductor section is of a maximum length.